Present vacuum plasma processing operations are generally performed in vacuum vessels at pressures in the range of 20 Torr or less, typically with pressures of a few microns (10 microns to 1000 microns). Two or more electrodes are mounted in the vessel and connected to a high-voltage power supply. A plasma is formed between the electrodes when a voltage exceeding the breakdown voltage is applied between the electrodes.
Although the voltages of plasma discharges may vary considerably according to gas species and the product of gas pressure and distance between electrodes (in accordance with the well-known Paschen Curve), in actual practice, most commercial discharge apparatus operate with power supplies having voltages in the range of a few hundred to a few thousand volts. However, even these voltages can constitute a hazard to operating personnel in the vicinity of the apparatus, especially since the vacuum vessels in which the discharges occur are often electrically conductive.
Consequently, for reasons of safety, the vacuum vessels are conventionally connected to ground potential. Most often the parts being processed are also connected to ground potential, although in some instances the parts are biased to a potential for certain process operations.
The power supply which energizes the electrodes and maintains the plasma discharge may be a direct current supply or may be an alternating current supply varying in frequency from a few Hertz to many megaHertz. However, in the case of both D.C. and A.C. supplies, it is the usual procedure to ground one electrode and connect the other electrode to the power supply. One side of the power supply is generally connected to ground potential while the other side is at a preset voltage with respect to ground.
There has been a problem with prior plasma generating apparatus due to the potential between the vessel walls and the high-voltage electrodes. In particular, when a plasma discharge is started between two electrodes (one of which is at ground potential and the other which is at a potential with respect to ground), the plasma may not be totally contained between the electrodes. In many instances a "stray" plasma discharge also forms between the high-voltage electrode and the vessel walls and between the lead-in wire to the high-voltage electrode and the vessel walls (at the power feedthrough into the vessel).
Such stray discharges waste processing power since the discharge is not contained between the electrodes and thus does not contribute actively to the process. In many instances thermal "hot spots" can be found on the vacuum vessel walls where stray, non-uniform discharges have occurred.
Accordingly, it is an object of the present invention to provide plasma discharge apparatus in which spurious discharges are eliminated.
It is another object of the present invention to provide plasma discharge apparatus in which discharges are confined mainly to the area between electrodes.
It is a further object of the present invention to provide plasma discharge apparatus which avoids the conventional loss of power due to spurious discharges.
It is still another object of the present invention to provide plasma discharge apparatus in which a more uniform plasma is obtained.